Actually, not all Pfhor do this, only certain ones. In a Marathon physics model, each monster/alien can be tagged as a 'Berserker' (located under 'Behavior Settings'). A monster with this tag turned on will show the behavior you've probably been noticing, which is to say, when significantly damaged, 'Berserker' monsters will attack anything in sight, regardless of its normal Friend-or-Foe settings.

As a general rule, only Major Pfhor have this tag actually turned on in the game. So, green and orange Fighters will not go berserk, but purple and blue ones will. This can be quite useful, as it allows you to let a whole group of enemies annihilate each-other while you simply wait around the corner listening or watching to determine when the killing has stopped.

Interesting. Is there a specific canonical reason for this trait? I come from a Halo (PC) background where the importance of hierarchy in the Covenant meant that Elites could regularly abuse Grunts at whim without consequences. Similar was the relationship between Brutes and Elites.

"as it allows you to let a whole group of enemies annihilate each-other while you simply wait around the corner listening or watching to determine when the killing has stopped."

Once I found there was a bunch of troopers behind the next door, far more than I could take on and survive. I opened the door, fired a grenade through and kept slamming the door closed. I finally opened it just in time to see the last two troopers annihilate each other. Amusing but a bit of an anticlimax.